1. Field of the Invention
This invention relates to the removal of solid products from a fluidized bed, particularly applicable to beds combusting carbonaceous material such as coals and char to form a product gas and agglomerated ash.
2. Description of the Prior Art
There is increased emphasis being placed upon systems which transform materials such as coal into other useful fuel forms, such as systems for coal liquifaction and gasification. Among such processes are those utilizing fluidized beds into which are fed particles of coal, or a coal derivative such as char or coke breeze to form a combustible product gas and product solids, primarily agglomerated ash particles with some char.
As the devolatization, combustion or other reaction takes place within the fluidized bed, the product or waste solids tend to congregate at the bottom of the containing vessel as a result of various operational phenomena such as agglomeration, and must be removed. It is known that various operational parameters can be adjusted to control the rate and degree of ash agglomeration and separation, such as the combustion operating temperature, the feed rates of various inlet gases and coal particles, and the type and size of particle initially fed into the bed. To the extent that these parameters can be adjusted to control ash formation, their adjustment indirectly represents one method of ash removal from the bed. Such adjustments are discussed in U.S. Pat. No. 3,981,690. Another manner in which to control ash removal is to adjust the flow rate of the gas, entering at the lower end of the containing vessel through a constriction such as a venturi, such that the heavier or denser agglomerated ash particles will overcome the resistance to downward motion provided by the gas, while lighter or less dense particles will remain above.
Another method of ash removal control is, as described in U.S. Pat. Nos. 2,534,051 and 2,677,604, to maintain the top level of a fluidized bed within a selected range. As the top reaches a prescribed upper level, a valve responds to allow removal of particles from the fluidized bed reactor.
While such control and removal techniques are functional, improvements can be made. For example, the venturi technique may be highly sensitive to changes in operational parameters, and control by upper level monitoring can be difficult since the level may not always be well defined. Additionally, the upper level is not necessarily representative of the amount of solid product buildup.
It therefore is desirable to provide alternate systems for withdrawal of solid products from fluidized bed reactors which preferably are accurate, automatic and simple in concept and structure.